1. Field of the Invention
The present invention relates to a preparation method for manufacturing an electron tomography (ET) sample and a method for constructing a three-dimensional (3D) image. In particular, the present invention relates to a preparation method for manufacturing an ET sample with embedded markers and a method for reconstructing a 3D image from series of tilted two-dimensional (2D) transmission electron microscopy (TEM) images.
2. Description of Related Art
The rapid advances in nanotechnology and the decreasing size of features in the microelectronics industry result in the need for advanced characterization with high spatial resolution in two and three dimensions. Therefore, the demands for physical failure and materials analysis using the transmission electron microscope or scanning transmission electron microscope are rapidly increasing: for instance thickness measurement, small defect identification, chemical element analysis, etc. It is well known that sample preparation is a crucial point for successful TEM or STEM analysis. Furthermore, some artifacts, such as amorphous layers and overlapping interfaces in small vias, will become more serious as the dimensions of the TEM sample become larger relative to the ever-shrinking device dimensions. ET provides a solution to characterize nano-scale devices by reconstructing a 3D image from a series of tilted 2D projections. Electron tomography includes four main steps: series of tilted 2D images acquisition, image processing, 3D reconstruction, and visualization. After acquisition, tilt-series alignments are performed to refine the relative image shifts and tilt-axis orientation using image-processing software. The acquired tilt-series must be precisely aligned with the tilt axis to minimize the blurring of small features and artifacts in the reconstruction. Two methods are used to align the tilt-series: cross-correlation and feature tracking. Feature tracking enables the correction of x-y shifts and the average tilt-axis orientation, and also allows image orientation and magnification changes to be measured and compensated. This provides a more accurate image alignment in the case of image distortion comparison with cross-correlation method. For prior art, there are two well-know methods to prepare an ET sample.
The first method is to place a flat and thin specimen onto the carbon-coated grids with fiducial markers. The fiducial markers (i.e., metal particles) are prior deposited on the carbon-coated grids. Then, the TEM specimen is placed on the carbon-coated grids above the metal particles. Those metal particles below the specimen are the references of series tilted 2D images alignment for feature tracking process. However, there is a gap between the specimen and fiducial markers. Therefore, the circular motion paths of fiducial markers are relatively longer during sample tilting compared with each point in the specimen resulting in a change in focus and image shift problems on those fiducial markers during series of tilted 2D images acquisition. Because of the specimen and fiducial markers are not at the same focus plane. That may make fiducial markers difficult to track or even missing in the worst case, especially with higher magnifications, thicker samples and higher tilt angles. In addition, the ET 3D images reconstructed from the traditional flat and thin sections are incomplete because the projection images can not be acquired from the range of directions lying close to the plane of the sample. The lack of information at high tilt angles is referred to as the missing wedge problem.
The second method is to manufacture a cylindrical or pillar specimen. The cylindrical or pillar specimen can be rotated and imaged in full 360 degree range (from 0° to 360°, or from −90° to +90°) with no change in the electron path length through the specimen. However, it is not easy for the user to deposit the metal particles (fiducial markers) onto the pillar specimen for feature tracking process.
The present invention is for preparing ET samples with gold beads inside to improve the feature tracking process and quality of 3D reconstruction.